The use of spin in the sport of tennis is a strategy employed by players at all levels. At intermediate and advanced levels, mastery of topspin and underspin offers a significant competitive advantage. For example, tennis players, who are able to hit the ball causing significant ball topspin, can aim the ball's trajectory well above the actual net (minimizing the error of the ball hitting the net) while relying on the spin to bring the ball down inside the opponent's boundary lines. This clearance allows players to hit the ball with greater speed with the confidence that it will land in the field of play. In addition, both topspin and underspin/slice will also cause difficulties for the opponent to respond. In the case of topspin, the ball will bounce and ‘jump’ off of the court making it difficult for the opponent to adjust. In the case of underspin, the ball will skid or die making it equally difficult for the opponent to adjust. It is accepted in the sport of tennis that those capable of consistently mastering topspin and underspin have reached a higher level of ability that will favorably impact their game.
Most tennis racquets are similar in shape and stringing to that shown in FIG. 4. The racquet shown in FIG. 4 has a typical string pattern 107 of 16 main strings and 19 cross strings (16×19). The main strings run in the direction of the Y-axis of the coordinate system 108 of FIG. 4, and the cross strings run in the direction of the X-axis. The Z-axis is normal to the string bed as shown in FIG. 4.
In the 1970s the spaghetti tennis racquet (or more appropriately named “the spaghetti strings”; almost any racquet could be strung using the spaghetti strings) offered a noticeable increase in spin rate over conventionally strung racquets for an equivalent tennis stroke. The spaghetti stringing technique was revolutionary and historically significant, and the present invention's design will be contrasted against the design of the spaghetti (2 expired patents define the spaghetti design in detail). The concept of the design of the spaghetti tennis racquet is shown in FIG. 1, FIG. 2 and FIG. 3. FIG. 1 shows a plan view of the spaghetti-strung racquet. The racquet frame 101 supports 6 cross strings 102. There are 2 pair groups of main strings (103 and 104) that are on either side of the cross strings. In FIG. 2, the front and back main strings (103 and 104) are shown as they lock into the slider-bars (105 and 106 in FIGS. 1 and 2). Most importantly, the 2 sets of main string are not interwoven with the cross strings as seen in more traditional stringing configurations.
Referring to FIG. 3A, the spaghetti is designed so that the front set of main strings (103), locked into the 4 slider bars (105), moves together as they slide on the 4 cross strings (102). Since they are not interwoven this movement is much easier than in traditionally strung rackets. This motion is roughly left<->right in FIG. 3A, or, more specifically, the X-direction of the coordinate system (108) of FIG. 3A (this X-direction is also called the 3 o'clock<->9 o'clock direction, and the Y-direction is also called the 12 o-clock<->6 o-clock direction; see FIG. 3A). On a smaller scale this motion also occurs with traditional stinging configurations by not interweaving the main and cross strings, the x-y motion for the spaghetti configuration is greatly amplified.
The back set of main strings (104) and slider-bars (106) function in the same way as the front assembly (although independent of the front assembly). Both sets of main string assemblies can flex for out-of-plane loading. For in-plane loading, only the side that contacts the ball flexes in the plane of the string bed.
When a ball is struck by a tennis racquet, both the ball and racquet are moving. It is common to investigate this impact by referencing the impact relative to the racquet frame: hence the racquet is fixed and the ball impacts it (relative velocities are used). This is demonstrated by the ball (110) in FIG. 3B, moving in the XZ plane of coordinate system 108, striking a racquet that is fixed to ground. The ball is coming in at an angle to the normal (Z-direction) of the racquet, and this simulates the real impact of a ball and racquet causing spin of the ball about the minus Y-direction. The vector 111 illustrates the path of the ball before impact. After impact, the ball rebounds with spin. The common explanation for the advantage of the spaghetti is that, during ball impact, the top main string assembly is pushed by the ball in the minus X-direction (the slider bars will slide on the cross strings). In addition, both the front and back main-string assemblies as well as the cross strings will simultaneously deform in the minus Z-direction). Energy is stored for both motions and then returned to the ball. The Z-direction energy rebounds the ball off the string bed; the X-direction energy allows the top main string assembly to rebound in the plus X-direction, applying a tangential force to the contact point of the ball. This tangential force applies a moment to the ball (about the minus Y-direction), and this causes the ball to spin about the minus Y-direction (right hand rule). Slow motion video during this contact shows the added spin may be due to this kick back tangential force, but it is also clear that the X-direction compliance of the main string assembly allows the ball to not slip on the string bed, causing added rotation. It will become obvious that, through a different mechanism, the present invention will also minimize the slipping of the ball on the string bed.
Another observation about the spaghetti is that the maximum spin that the spaghetti can offer is directly related to the directional impact of the ball on the racquet. Referring to FIG. 3A, let the angle that the ball makes with the Z-axis be constant. But let the ball approach the racquet in the YZ plane. It is obvious that the spaghetti loses its advantage here since the in-plane stiffness of the main-string assembly in Y-direction is significantly stiffer than in the X-direction. Any direction other than the biased XZ plane will have less spin effectiveness; and such a direction occurs in actual play when a ball is struck when the Y-axis of the spaghetti racquet is not parallel to the tennis court. It will become obvious that, unlike the spaghetti system, the present invention is not dependent on the angle of approach.
Another problem with the spaghetti is that the in-plane and out-of-plane stiffness was not controlled. Most tennis players (pros and amateurs alike) hit with racquets whose out-of-plane string bed stiffness is 140/150 lbs/in to 250 lbs/in. A stiffness softer than this makes the ball “trampoline” off the string bed, which both significantly hampers control and significantly hampers keeping the ball “in the court”; and stiffness higher than this make the racquet hit like a board with a significant loss in power. The spaghetti system offers out-of-plane stiffness in the order of 90/100 lbs/in, making it almost impossible to control if the motion of a player's stroke did not lend itself to generating topspin. Because of the double string assembly and the plastic roughed-up inserts 103 and 104 of the spaghetti design shown in FIGS. 1 thru 3A, the spaghetti system no longer meets United States Tennis Association and International Tennis Federation rules for a strung tennis racquet to be used in sanctioned tournament play. It will become obvious that the present invention can provide an in-plane and out of plane stiffness better suited to current expectations.
Tennis players and tennis manufacturers, over the last several years, have found another way to help increase ball spin: open string patterns. FIG. 4 shows a racquet that is strung with a conventional stringing pattern (16 mains×19 cross). FIG. 5 shows the same racquet strung with an open string pattern of 16 main strings (110) and 10 cross strings (109); and FIGS. 6 and 7 illustrate a close-up comparison of these string patterns. There are other open string patterns that have significantly less strings, but the principle on which the open string pattern causes increased top spin is the same: the string kickback and the in-plane compliance of the main strings is the key. As the ball strikes the open string bed, in exactly the same manner outlined previously for the spaghetti, the main strings slide on the cross strings, and then rebound. Once again, slow motion video during this contact shows the added spin may be due to this kick back tangential force, but it is also clear that the X-direction compliance of the main string allows the ball to not slip on the string bed, causing added rotation. With less cross strings interweaving the main strings are able to move more than traditional stringing patterns, though still less than that of the spaghetti system.
The open string pattern has several problems in its use. The open string pattern has the same directional limitation that was explained in the spaghetti system: an open strung racquet making an angle to the tennis court as it impacts the ball will get only a partial advantage of the spin generated by the open pattern (compared to the same racquet, same conditions, but the racquet is swung parallel to the court). Another disadvantage of the open string pattern racquet is the significantly increased wear of the string bed causing a shorter string life. Since the movement of the main strings sliding over the cross strings is fundamental to the advantage of the open string system, it is no surprise to see the cross strings essentially “sawing” the main strings in half. And this is indeed the case, where the more effective the open string pattern is to cause increased spin, the shorter the main string life. In addition, this frictional sliding reduces the amount of in-plane-motion returnable energy that is available for spin generation. It will become obvious that the present invention overcomes these limitations in the open stringing pattern.
A review of prior art shows previous patents that include an inner and outer frame construction. FIG. 8 serves as a pictorial example of such a dual frame construction: the inner frame 201 supports the string bed, isolators 202 will structurally integrate the inner and outer frames, and the outer frame 203 completes the racquet and delivers the handle interface to the tennis player. The isolators could be a collection of the discrete isolators as shown in FIG. 8, or a continuous system illustrated by a rubber tube or a continuous leaf spring. In the case of one patent, the inner frame is essentially integral with the outer frame; hence it is not isolated. In another case there is a rubber tube that holds the inner and outer frames together. The purpose of the both patents is to easily change the strings/inner-frame from the outer frame. This allows the quick replacement of a pre-strung inner frame. Other prior art uses an inner and outer frame construction to help minimize vibration of the racket upon impact often linked to tennis elbow. In none of the prior art is there any claim or objective associated with added topspin or underspin. There is also no discussion of: i) the weight of the inner frame; ii) using/adjusting the in-plane and/or out-of-plane stiffness of the isolators to increase spin; iii) using/adjusting the isolation system to improve the accuracy of the directional trajectory of the impacted ball; iv) using/adjusting the isolation to offer rotational independence of ball impact (occurs when the racquet's Y-direction is not parallel to the court); v) controlled stringing procedures to reduce inner frame stress and buckling; and vi) extended string life.